Sentence Correction

Construction of the Roman Colosseum, which was officially known as the Flavian Amphitheater, began in A.D. 69, during the reign of Vespasian, was completed a decade later, during the reign of Titus, who opened the Colosseum with a one-hundred-day cycle of religious pageants, gladiatorial games, and spectacles.

A. which was officially known as the Flavian Amphitheater, began in A.D. 69, during the reign of Vespasian,

B. officially known as the Flavian Amphitheater, begun in A.D. 69, during the reign of Vespasian, and

C. which was officially known as the Flavian Amphitheater, began in A.D. 69, during the reign of Vespasian, and

D. officially known as the Flavian Amphitheater and begun in A.D. 69, during the reign of Vespasian it

E. officially known as the Flavian Amphitheater, which was begun in A.D. 69, during the reign of Vespasian, and

OAC

Doubt: In OA which was officially known as the Flavian Amphitheater is a modifier, modifies the Roman Colosseum right?

The subject of the sentence is CONSTRUCTION, which has two verb Began and Was completed right? So why the BEGAN is in Active Form and WAS COMPLETED is in Passive Form? Both must be parallel form. Also please explain why AND is necessary in OA?

Many thanks in advance,

Kavin